This is part one of a two-part study that investigates water single-phase and two-phase pressure drop across a staggered array of circular micro-pin-fins. This paper presents results of isothermal liquid single-phase flow, and part two adiabatic liquid-vapor two-phase flow. The micro-pin-fins are 180 mu m in diameter, 683 mu m in height, and 399 mu m in both longitudinal and transverse pitches. Seven temperature levels from 23 to 80 degrees C, and seventeen maximum mass velocities for each temperature, ranging from 133 to 1538 kg/m(2) s, were tested. The resulting Reynolds number ranged from 26 to 776. Pressure drop across the micro-pin-fin array was measured, and friction factor calculated. The existing friction factor correlations for micro-pin-fin arrays, conventional-size pin-fin arrays, and tube banks were examined and found unable to predict the data. A numerical simulation was performed, and a good agreement was found between the experimental data and numerical results. A new friction factor correlation was developed that is used in part two of the study on calculating liquid-vapor two-phase pressure drop. (C) 2015 Elsevier Ltd. All rights reserved.